In the production of filter tow for use in making filter rods for the cigarette industry, the tow is laid in so-called “filling cans”. During this process, the filter tow is distributed in uniform layers over the cross-sectional area of the can by the movements of a laying unit, which moves alternately in the lengthwise and crosswise direction. As a result, a large number of layers are laid on top of each other until the filter tow package has reached the desired weight and height in the can. Package weights of several hundred kilograms are conventional in this area. A highly compressed bale and a process for the optimal filling of a can for the purpose of avoiding consequent processing problems is described in WO 02/32,238 A2.
The content of the can which has been filled in this way is then compressed in the direction in which the layers were superimposed. After it has been compressed, the filter tow package is wrapped with packaging material while still inside the pressing device and therefore still under compressive stress. The pressing device is then opened completely, so that the filter tow package, now called the “bale”, is held together by the packaging material. Conventional packaging materials include cardboard, which is held mechanically together by strapping or by an adhesive, and synthetic fabric, which is closed by, for example, a Velcro fastening. An example of a glued package is described in German Utility Patent No. 76-35,849.1. Information on a filter tow package wrapped with synthetic fabric can be found in the company prospectus “Some Useful Information about the Reusable Packaging for Rhodia Filter Tow”, published by RHODIA Acetow GmbH, Engesserstrasse 8, D-79108 Freiburg. The two latter types of packaging require no additional strapping.
The types of packaging described above which do not make use of any strapping suffer from the problem that, after the pressure on the bale has been released at the end of the pressing operation, the elastic restoring force of the compressed filter tow leads to a pressure on the packaging, this pressure being exerted primarily in the direction opposite that in which the bale was compressed. This leads to an increase in the volume of the package and thus to undesirable bulges at the top and bottom of the bale. If the measures described in WO 02/32,238 A2 are taken, these bulges do not interfere with the intended use of the filter tow, but they do prevent the filter tow packages from being stacked securely. This problem is solved in the state of the art either by stacking the bales on their sides or by the use of special pallets, such as those described in the Rhodia publication cited above. Problems associated with the bursting-open of the packages because of excessive internal pressure also occur frequently.
A solution to the difficulties associated with strapping is described in U.S. Pat. No. 4,577,752. In cases where filter tow which has been packaged with straps is used as intended, the bulges are less of a problem than the constrictions, which cause the variations in puff resistance described in WO 02/32,238 A2. And even strapped bales can burst open. It is also standard practice in the packaging of filter tow to use liners between the filter tow and the above-mentioned mechanically supportive packaging materials. The liner protects the filter tow from contamination, especially from odor contamination, and from the diffusion of water vapor into and out of the package. The liner usually consists of three pieces, which are laid loosely inside the external packaging.
The disadvantages of the transport packaging normally used today have already been discussed above in the description of the state of the art. It is especially the bulges at the top and bottom of the bales which interfere with transport of multiple layers. This problem has been solved in the past by transporting the bales not in their so-called working position but rather in a sideways storage position. Two additional work steps are required to do this, however; namely, the bale must be turned 90° before transport and then turned back into the working position after transport. The constrictions which are formed by strapping are also a source of trouble. Even when the bale is used as intended, these constrictions cause considerable variations in the puff resistance of the filter rods produced from the filter tow. More than 5% of the filter rods produced from a bale are affected by these variations. The greater the packing density of the bale, the greater the severity of these two problems. The problems occur as soon as the packing density exceeds 300 kg/m3.